Ceramic vibration plates for mid and high range speakers are generally configured as domes as shown in FIG. 1, wherein a vibration plate 1 is driven by a coil assembly 10 in the usual manner. Conventional methods for manufacturing such ceramic speaker vibration plates include machining, ceramic powder molding, and projection molding.
FIGS. 2(a) and 2(b) show the sequential steps involved in powder molding, wherein reference numeral 2 designates an outer mold, 3 is an upper punch, 4 is a lower punch, 5 is a spring, and 6 is a ceramic powder. In this method the powder 6 is placed in the cavity formed by the outer mold 2 and the lower punch 4 as shown in FIG. 2(a), whereafter the upper punch 3 is driven downwardly to compress the ceramic powder as shown in FIG. 2(b). The compressed powder is then sintered to harden it into the vibration plate 1.
FIGS. 3(a) and 3(b) show the sequential steps involved in projection molding, wherein reference numeral 7 designates a gun for projecting a ceramic powder, 8 is a mother model for the vibration plate, and 9 is a projected film. In this method the gun 7 projects the ceramic powder onto the surface of the mother model 8 to build up the film 9 as shown in FIG. 3(a), whereafter the mother model is withdrawn as shown in FIG. 3(b) to leave the vibration plate 1.
In the powder molding method the sintering time is undesirably long, and owing to the domed shape of the plate the compressive force is not uniformly distributed which results in the density of the ceramic powder being non-uniform. This adversely affects the speaker characteristics.
In the projection molding method the film 9 is generally porous, and it is difficult to form a sufficiently thin film. This problem is particularly acute for modern light weight speakers wherein the vibration plate is desirably only 30 .mu.m.about.100 .mu.m thick.
The machining method similarly presents problems because it is difficult to grind down the hard and brittle ceramic starting material or block, the apparatus needed to machine a dome shape is complex and costly, and considerable wastage of the expensive ceramic material is involved.
Other methods include chemical vapor deposition and ion plating, but they are limited to only certain types of ceramics and it is difficult to form a sufficiently thin film.